Subjects aged 40-44 with blood pressures in the upper 5th percentile will be ascertained from the records of a health testing centre and matched with controls below the lower 10th percentile of blood pressure. 100 cases and 100 controls will voluntarily attend, with their families, for the measurement of blood pressure, immunoglobulins, immune complexes, autoantibodies, HLA antigens and other genetic markers. Other personal and behavioural characters will be measured, and related to the distribution of blood pressure within and between families. The results will be analysed to test the major hypothesis that essential hypertension is partly determined by genes in linkage disequilibrium with the major HLA loci in man. BIBLIOGRAPHIC REFERENCES: MATHEWS, J.D. (1977) Assessment of HLA Heterogeneity: Alternative Tests of the Null Hypothesis in Disease Association Studies. Proceedings 1st South East Asian and Australasian Tissue Typing Workshop. pp. 119-127. (University of Western Australia). UNGAR, B., MATHEWS, J.D., COWLING, D.C. and TAIT, B.D. (1977) HLA Patterns in Pernicious Anaemia. Bit. Med. J., in press.